鄂尔多斯盆地华池地区三叠系延长组长7段富有机质页岩岩相特征

Lithofacies features of organic-rich shale of the Triassic Yanchang Formation in Huachi Aera,Ordos Basin

鄂尔多斯盆地三叠系延长组长7油层组发育富有机质页岩和泥岩,目前已成为鄂尔多斯盆地页岩油气勘探的重要层位。综合岩心精细描述、薄片微观结构观察和粒度分析结果,将富有机质页岩和泥岩划分为黑色页岩岩相、纹层状泥岩岩相、粉砂质泥岩岩相。粉砂质泥岩岩相的脆性矿物含量高于黑色页岩岩相和纹层状泥岩岩相,后者比前者具有更高的黄铁矿含量,3种岩相的黏土矿物含量较接近。黑色页岩岩相有机质丰度高,有机质类型以I型干酪根为主,是最有利的富有机质岩相类型,其次为纹层状泥岩岩相。长7油层组沉积时期水体为分层的厌氧环境,有利于有机质的保存,低等水生生物是主要的母质来源。黑色页岩岩相和纹层状泥岩岩相形成于安静、缺少陆源碎屑影响的深湖环境下;粉砂质泥岩岩相形成于受陆源碎屑频繁影响的环境,有较多的陆源高等物质输入湖盆中,造成有机质类型复杂。将岩心精细描述和测井信息紧密结合,揭示了各类岩相纵向发育特征。受基准面变化和沉积物供给能力影响,长73以黑色页岩岩相和纹层状泥岩岩相为主,长72和长71以纹层状泥岩岩相和粉砂质泥岩岩相为主。长73暗色泥岩和页岩的岩相类型好,发育页理缝和纹层缝,有机质类型好、丰度高,是最有利的页岩油气勘探目的层系。

Organic-rich shale and mudstone developed well in Chang-7 Member of the Triassic Yanchang Formation,which has become an important prospecting target for shale gas and shale oil.Based on fine description of core,micro-structure observation of thin section and grain analysis,three kinds of lithofacies of shale were determined,including black shale,laminar mudstone,and silty mudstone.Brittle minerals content of silty mudstone is higher than that of black shale and laminar mudstone.Pyrite content of the latter is higher than that of the former,and clay minerals content of the three lithofacies are nearly the same.Black shale is the most favorable lithofacies which has high organic abundance and greatest kerogen type（mainly type I）.Laminar mudstone is the second favorable lithofacies.During the deposition of Chang-7 Member,the layered anaerobic environment was beneficial to the preservation of organic matter.Lower grade aquatic lives were the main biological precursor of organic matter.Black shale and laminar mudstone developed in deep lacustrine environment which was quiet and not influenced by terrigenous coarse grains.However,silty mudstone developed in the environment that was influenced frequently by terrigenous coarse grains.As a result,organic matter types of silty mudstone are diverse and complicate.With the combination of fine description of core and logs,the vertical development characteristics of each kind of lithofacies were uncovered.Influenced by baselevel changing and sediment supply,the third section of Chang-7Member（C73）is composed of black shale and laminar mudstone,however,the second section and first section of Chang-7 Member are mainly composed of laminar mudstone and silty mudstone.Black mudstone and shale in C73 have better lithofacies that bear lamellation fracture and lamina facies,better organic matter type and high abundance.C73 is the best prospect target for shale oil and shale gas.